1. Field of the Invention
The present invention relates to a method of forming a polycrystalline silicon (i.e., polysilicon) film in a process of manufacturing an LCD (Liquid Crystal Display).
2. Description of the Related Art
An LCD using a TFT (Thin-Film Transistor) has received a great deal of attention because the LCD provides very excellent picture quality.
An LCD substrate used in the LCD is schematically shown in FIG. 1. A large number of pixel units U each of which is obtained by combining an TFT 102 to a pixel electrode 103 connected to the drain of the TFT 102 are arranged on a glass substrate 104. For example, several hundred thousands square pixel units U whose side has a length of about several hundreds .mu.m are arranged. A transparent electrode 105 commonly used for the pixel units U is formed to oppose the pixel electrode 103 such that a liquid crystal 106 is formed between the pixel electrodes 103 and the transparent electrode 105, thereby forming a pixel portion 100 as shown in FIG. 2.
A large number of packaged IC chips 107 are arranged on the glass substrate 104 along the peripheral edge of the pixel portion 100. A driver unit 101 is formed by the IC chips 107. These IC chips are independently formed and then adhered to the glass substrate 104. The terminals of each of the IC chips 107 are connected to a source bus and a gate bus connected to each of the pixel units U of the pixel portion 100. For example, the number of gate buses and the number of source buses may reach 400 to 1,920. For this reason, a large number of wiring steps are required to connect the IC chips 107 to the pixel units. This is a factor of increasing the cost of an LCD.
The TFT of each pixel unit does not require its high-speed operation because the TFT causes the pixel of the pixel unit to be displayed as an image. For this reason, an amorphous silicon layer can be used as a semiconductor layer for the TFT. However, each transistor of the driver unit must have an operation speed considerably higher than that of the TFT because a circuit requiring a high-speed switching operation must be mounted on the driver unit. Therefore, a polysilicon film having a field-effect mobility higher than that of an amorphous silicon layer must be used as the semiconductor layer of the driver unit.
The film formation process must be performed at a temperature of about 600.degree. C. or more by, e.g., low-pressure CVD (LPCVD) to obtain a polysilicon layer. However, an inexpensive glass substrate has a thermal strain point of about 600.degree. C. A glass substrate which can withstand a high temperature of about 600.degree. C. is expensive.
Under the above circumstances, a common semiconductor layer which is used for constituting a driver unit, a pixel portion, and a wiring layer cannot be formed on the same glass substrate at the same time. Therefore, a cumbersome and costly operation of adhering IC chips for a driver unit to a glass substrate is inevitable at present.